The present invention relates to two-piece fasteners and more particularly to swage type fasteners having improved fatigue, tensile strength and retained clamp or preload characteristics.
The present invention generally relates to two-piece swage type fasteners or lockbolts of the type illustrated in U.S. Pat. No. 3,915,053 to J. Ruhl, issued Oct. 28, 1975, U.S. Pat. No. 2,531,048 to L. Huck, issued Nov. 21, 1950 and to U.S. Pat. No. 4,472,096 to J. Ruhl and R. Dixon, issued on Sept. 18, 1984. All of the above can be considered as prior art relative to the present invention.
In many commercial applications, two-piece threaded fasteners are used and are designated by a grade indicative of a particular strength level. For example, a 1/2-13 UNC Grade 8 bolt is a bolt for use in a nominal 1/2" diameter opening and has a 13 thread per inch coarse thread. The latter threaded fastener has a standard thread form which is designed to fail in a tensile mode across the diameter of the pin threads at a minimum preselected tensile load on the fastened joint. Similarly, a 1/2-20 UNF Grade 8 bolt will have a fine thread of 20 threads per inch of a standard configuration and will have somewhat different load and strength characteristics. Along the same line, a Grade 5 bolt will have lower strength characteristics than the Grade 8 and a Grade 9 will have higher strength characteristics. Often, a threaded fastener in order to provide as much clamp load as possible to the workpieces of the fastened joint will be torqued to the point where the bolt yields in tension. In many applications, however, where yielding is unacceptable the magnitude of the clamp load must be significantly reduced. In either case, however, because the pin is subjected to a combined stress, which includes the torsional stress from the applied torque and tensile stress from the resultant tension applied to the bolt, the magnitude of clamp load which can be realized is less than that if the fastener could be set without torque such as in a swage type system. In addition, of course, the torque type fasteners have the inherent problem of friction which also detracts from the amount of clamp load that can be realized from the applied torque.
In the present invention, however, a two-piece swage type fastener concept is provided which when constructed to provide a given design tensile load will have significantly improved fatigue and retained preload characteristics relative to its torque applied, threaded counterpart.
Thus, in the present invention, the fastener comprises a pin and a collar adapted to be swaged into lock grooves in the pin. The lock grooves in the pin, unlike prior lock grooves, are very shallow and are constructed to have roots of a simulated streamlined shape. The shallow grooves and simulated streamlined shape contribute to a resultant fatigue life which is vastly superior to that of the comparable threaded fastener. At the same time the collar is provided with a predetermined wall thickness, and hence volume, not only to provide an over fill condition of the lock grooves during swaging but also to provide a retained clamp having a magnitude which is a high percentage of the installation load and of the yield strength of the pin in tension across the lock grooves. Thus the collar wall thickness is selected to enhance retained clamp and to provide sufficient hoop stiffness to inhibit spring back of the collar material out of the shallow lock grooves in the pin. The subject fastener will also provide clamp loads which are significantly higher than those achievable with the threaded counterpart because of the absence of the applied torque (and resultant friction) required for the threaded system.
In the present invention the design failure load can provide a failure in shear across either the shoulders of the pin lock grooves or the shoulders defined by the swaged collar. In other words, a relatively high strength pin is used which will not fail across the engaged grooves at the lower magnitudes of applied tensile loads. Thus, with a given pin, the magnitude of available design tensile load can be increased or decreased simply by varying the length of the collar and hence by varying the number of pin and collar shoulders that are supporting the tensile load. This selective variation can occur until the appropriate number of engaged shoulders are provided whereby failure under tensile loading will occur as a tensile failure across the diameter of the pin.
In one form of the present invention the resultant preload applied to the pin is slightly below that load at which yielding would occur in the pin lock grooves. The collar is slightly longer than that required to provide engagement of the appropriate number of pin and collar shoulders to hold that preload; in this case the failure at the design tensile load will be in shear across the collar and/or pin shoulders. By simply increasing the length of the collar the number of engaged shoulders will be increased whereby failure will occur at a higher tensile load in tension diametrically across the pin lock grooves. In either event, the result will be a retained preload of a high magnitude on the fastened joint. In fact this retained preload for a system of ferrous materials will be at around 85% to around 95% of the yield strength of the pin in tension across the lock grooves. A similar percentage of retained clamp load of the latter magnitude is not readily attainable by the noted threaded systems without undesirably high installation and residual stresses because of the torsional stress applied in torquing the fastener as well as other factors.
Note that when the high preload levels are considered in conjunction with the advantages of lock grooves which are considerably shallower than the threads or grooves in the comparable threaded system, the attainable improvement in fatigue and retained stress is even more significant. As will be seen, some additional gains can be secured by providing the lock grooves to be helical in shape rather than annular. Here the tensile strength and fatigue life of the pin will be somewhat greater since the helical shape will provide a greater diametrical lock groove dimension; this results in a greater effective cross-sectional area than that provided across the minimum root diameter where an annular construction is used.
Thus the fastener of the present invention will not only provide significantly improved fatigue characteristics but will also provide a very high clamp load, i.e. at around 85% to around 95% of the yield strength of the pin. In one form of the invention the fastener can provide a tensile load carrying capacity up to the tensile strength of the pin across the lock grooves which, because of the shallow grooved structure, will be of a maximized magnitude relative to the pin diameter.
Thus it is an object to provide a novel two-piece swage type fastener with a pin having relatively shallow lock grooves of a simulated streamlined root configuration whereby fatigue life of the installed fastener is improved.
It is another object of the present invention to provide the pin with shallow grooves of the above described type whereby the clamp load of the fastened joint can be maximized relative to the installation load and the tensile yield strength of the pin.
It is still another object of the present invention to provide a novel two-piece swage type fastener having a collar of a preselected wall thickness for providing a magnitude of retained clamp load which is a high percentage of the installation load applied to the fastened joint.
It is another general object of the present invention to provide a novel two-piece swage type fastener.